(i) ${\mathit{C}\mathit{r}}_2{\mathit{O}}_7^{2-}+6{\mathit{F}\mathit{e}}^{2+}+14{\mathit{H}}^{+}\to 6{\mathit{F}\mathit{e}}^{3+}$ $+2\mathit{C}{\mathit{r}}^{3+}+7{\mathit{H}}_2\mathit{O}$
(ii) $2{\mathit{C}\mathit{r}\mathit{O}}_4^{2-}+2{\mathit{H}}^{+}\to \mathit{C}{\mathit{r}}_2{\mathit{O}}_7^{2-}+{\mathit{H}}_2\mathit{O}$
(iii) $2{\mathit{M}\mathit{n}\mathit{O}}_4^{-}+5{\mathit{C}}_2{\mathit{O}}_4^{2-}+16{\mathit{H}}^{+}\to 2{\mathit{M}\mathit{n}}^{2+}$ $+10{\mathit{C}\mathit{O}}_2+8{\mathit{H}}_2\mathit{O}$
OR
(i) Transition metals shows variable oxidation state because they have tendency to loose electron from their penultimate $\mathit{d}$-subshell and exhibit various oxidation state.
(ii) $\mathit{Z}\mathit{r}$ and $\mathit{H}\mathit{f}$ have almost same radii because of lanthanoid contraction. Filling up of electrons in $4\mathit{f}$ series shows poor shielding effect.
(iii) $4\mathit{d}$ series and other transition metals act as catalyst because they participate in various type of chemical reactions i.e.,